In this image, you can see the underlay stitches used to prepare the fabric for the stitches that are about to be sewn.
Using proper underlay stitches is essential to quality embroidery. If you can see the shirt, towel or hat fabric through the embroidery stitches, underlay stitches can be added to solve the problem.
Underlay stitches can hold the fabric down so it won’t peak through the stitches.
Underlay stitches also sandwich the fabric to the backing and underlay stitches prepare the fabric for the stitches that are about to be laid down.
Using the proper underlay takes your embroidery from semi-pro to professional.